Method of nondestructive tigtness testing based on gas discharge visualization

ABSTRACT

A method of nondestructive noncontact tightness testing based on gas discharge visualization. The invention relates to tightness testing methods and can be used for testing weld seams of articles inside of which there is a working medium (matter), for example, of chemical power sources. The method includes subjecting the article being tested to a high voltage pulsed electric field formed between the positive electrode and a metallized layer of the article casing, recording on a photo carrier, for which purpose the article is placed on a dielectric plate arranged on a positive electrode, and its surface area is selected in the ratio to the article surface area not less than 7:1, and is additionally subjected to an electric field of the preliminary polarized dielectric plate with a surface area ratio to the surface area of the positive electrode of 1.2:1, and is symmetrically arranged on the positive electrode relative to its center between the latter and the article being tested.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of Ukrainian Application No.2003077007 Filed on Jul. 25, 2003; now Patent No. 64623 issued on Sep.11, 2003, the entirety of which is incorporated herein by reference.

FIELD OF INVENTION

The invention relates to tightness testing and can be used fornondestructive testing of the quality of weld seams in articles thatcontain a working medium welded into a metallized dielectric casing(article jacket).

BACKGROUND OF THE INVENTION

Known in the art is a method of the article tightness testing [4]wherein a closed space is formed in the testing zone that is formed bythe outer surface of the article and the transparent electrode,whereafter the article and the electrode are subjected to a differenceof potentials that corresponds to the threshold value of the gasdischarge formation, and the inner surface of the article is blown by atesting gas that, while diffusing through the fault into the closedspace causes the formation of a discharge electron avalanche. Arecording device, for example a photo camera is positioned on the sideof the transparent electrode to record the discharge. The informationcarrier in the known method is the photo image of the discharge electronavalanche whose shape allows to evaluate the spatial inhomogeneity ofthe gas emission through the surface of the article and to determine thetightness failure range and location.

A limitation of the known method is the necessity to apply blowing bythe testing gas that complicates the testing process and excludes itsapplication for the articles whose inner volume is filled with a workingmedium featuring specified properties.

At present there are known methods of nondestructive testing [2, 3] thatdo not require any testing gas usage and that allow revealing of articlefaults on the basis of gas discharge visualization. According to thesemethods the article being tested and the recorder in the form of aliquid crystal cell are placed between the electrodes, while a highvoltage is applied to the electrodes, and the article is subjected to aformed electric field while its strength vector being normal to thearticle surface. The testing results are indicated by the image of thegas discharge process recorded by the registration unit.

The fault availability is indicated by a change in the form of the gasdischarge image caused by a distortion of the normal component of theelectric field within the fault area. To improve the indicatingpotential of the testing the air gap between the article and therecording unit is maintained within the 50-100 μm range. The characterof the gas discharge image is a function of the thickness and dielectricpermeability of the layers that confine the gas discharge gap, while theshape is determined by the shape of the article and is comprised ofseparate film exposure spots whose quantity per surface area unit isdetermined by the amplitude and number of pulses during the exposure toa high voltage electric field. The film exposure is caused by theavalanche-like discharge processes that linearly propagate along theelectric field lines of force from the separate points of the object tothe image recording unit. The discharges appear at random points of theobject uniformly over the whole surface area of the gap. The presence offaults on the surface or in the volume of the article results in adistortion of the electric field symmetry in the gas discharge gap. Thisis reflected in the shape of the gas discharge image. In such case anincrease in the electric field strength causes a preferential and mostintensive development of avalanche-like discharges in this field, whileits decrease leads to discharge weakening. Therefore the faulty areawill be exposed on the recording unit either lightly or stronger incomparison to the background, that is to the faultless areas. Thetesting sensitivity is a function of the electric field strength in thefaulty and faultless areas.

A common limitation of the conventional methods is their low sensitivityin testing of articles that have outer electroconducting layers thatdrastically reduce the indicating potential of the testing according tothe value of the change in the normal electric field component withinthe faulty area of the article.

There is a known method of recording air discontinuities in a solid body[5] wherein an article being tested is placed between the electrodes towhich high voltage is supplied, while a glow is caused in the dischargegap, the character of the glow reflecting the inner structure of thearticle. The discharge glow is recorded on the photo carrier. Todetermine the size of the air space in the direction of the electricfield lines of force the discharge photography is carried out in twostages: while a picture is taken at the threshold of the dischargeignition, the value of the applied voltage is recorded, whereupon thevoltage on the electrodes is continued to be increased up to such avalue at which a discharge occurs in the air volume of the fault that isreflected on the film. In this case the fault image will be brighter orsuch as in the faultless areas.

By recording the value of the applied voltage and comparing it with thevoltage at which the first picture was obtained information is achievedon the fault availability and location in the direction of the electricfield lines of force.

A disadvantage of this known method is the high error in determining thevoltage values of the discharge ignition in the fault space thatsubstantially lowers the testing sensitivity.

There is also known a method of nondestructive testing for faultyadhesion in the thin layer metal-dielectric arrangement [6] in which themetallic substrate of the article is used as one of the electrodes. Overthe dielectric layer the photo carrier is laid with the emulsion layerto the article surface. The whole system is pressed with a roller thatis rolled along the article surface, while voltage is fed from a highvoltage impulse generator to the roller functioning as a positiveelectrode. After exposure the photo material is processed using aconventional photographic printing method. In such case an appearance ofa fault that is of an air interlayer between the metallic substrate andthe dielectric coat of the article leads to a reduction in the fieldstrength on the outer surface of the article and, correspondingly, toweakening of the discharge that appears in the faulty area. These areaswill be brighter in comparison to the faultless areas. The sensitivityof the known testing method is determined by the character of thedistribution of the normal electric field component above the surface ofthe article being tested and by the relationship between the faultyadhesion value and the dielectric coat thickness.

A disadvantage of the known method is the necessity to form a gasdischarge gap on the side of the dielectric layer of the article, thusprecluding the possibility of its usage for articles with a metallizedlayer, and the sensitivity of this method will be low as only the normalcomponent of the electric field will operate within the non-adhesionarea.

SUMMARY OF THE INVENTION

The proposed method is based on gas discharge visualization of the innerstructure of the article being tested by applying thereto a high voltageelectric field. This phenomenon is known as the Kirlian effect. Thestudies carried out by several authors [1, 2, 3] showed that a necessaryand sufficient condition for the realization of the gas dischargevisualization is the provision of a discharge gap around the article,excitation and recording of the gas discharge image generated thereinwhen the article is subjected to a high voltage electric field.

The main purpose of the invention is to improve the method of tightnesstesting by means of gas discharge visualization through forming anoptimum directed gas discharge process independently on the conductivityof the outer layer of the article being tested. This effect is achievedby subjecting the article to the action of a superimposed electric fieldhaving both normal and tangential components.

With the purpose of obtaining a new useful result the proposed method ofnondestructive non-contact tightness testing based on gas dischargevisualization includes subjecting the article being tested to a highvoltage impulse electric field formed between a positive electrode and ametallic layer of the dielectric article casing, and recording on aphoto carrier of the image that appears as a result of the gas dischargeprocess, wherein according to the invention an article is placed on adielectric plate arranged on a positive electrode whose surface area isselected in relation to the surface area of the article in the ratio notless than 7:1, and the article is additionally influenced by an electricfield of the preliminary polarized dielectric plate arranged between thepositive electrode and the article being tested, while the ratio betweenthe surface area of the dielectric plate and the surface area of thepositive electrode is selected within 1.2:1, and the plate is arrangedsymmetrically of the center of the positive electrode. The dielectricplate is preliminary polarized by not less than three high voltagepulses of 100-120 μsec modulated by 200-300 kHz frequency.

The different surface areas of the article being tested, the positiveelectrode and the dielectric plate provide conditions for the appearanceof the tangential component of the electric field. It is this componentthat provides the formation of the discharge glow in the horizontalincomplete fusion areas, and in other horizontal faults of the articlethus a new property of the proposed method is provided for achieving theuseful purpose.

DESCRIPTION OF DRAWINGS

FIG. 1. Schematic diagram for tightness testing of an article with anoperating medium welded in a metallized polyethylene casing.

FIGS. 2, 3, 4. Distribution of the electric potential on the coloredimage and of the electric field strength vector in the active zone (2 a,3 a, 4 a), and numerical distribution of the electric field tangentialcomponent on the dielectric plate surface when the lower electroderadius equals 2 cm (FIG. 2.), 4 cm (FIG. 3), 6 cm (FIG. 4)correspondingly.

FIG. 5. Influence of an increase in the lower electrode surface area onthe tangential component value of the electric field on the dielectricplate surface.

FIG. 6. Gas discharge visualization image on an X-ray film showing thestructure of the article being tested that contains artificial defectsformed by 100-300 mm diameter steel wire pieces that were inserted inthe welding zone of the casing and taken out of the weld seam area afterwelding.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 shows a schematic diagram for article tightness testingcontaining a high voltage pulse generator 1, article metallizedpolyethylene casing 2, article operating medium 3, article weld seamarea, discontinuity (faulty weld) air gap leading to tightness failure,X-ray film 6, dielectric polarized plate 7, positive electrode 8.

To prove the rightfulness of the accepted relations between thedimensions of the elements in the control circuit, we shall use theLaplace's equation in cylindrical coordinates while presenting all thecircuit elements in the calculation circuit as cylindrical for thatpurpose. The circuit section shown in FIG. 1, seems to remain unchangedin such case.

We write the boundary problem in conformity with the figure:$\begin{matrix}{{\frac{\partial^{2}{V_{i}( {r,z} )}}{\partial r^{2}} + {\frac{1}{r}\frac{\partial{V_{i}( {r,z} )}}{\partial r}} + \frac{\partial^{2}{V_{i}( {r,z} )}}{\partial z^{2}}} = 0.} & (1)\end{matrix}$

Equation (1) is Laplace's equation Δφ=0 written in the cylindricalsystem of coordinates taking into regard the cylindrical symmetry of thetask accepted for the calculations. Values r,φ,z—are coordinates in thecylindrical system of coordinates, value ε is the dielectric constant,b, a, l are dimensions of electrodes and dielectric thicknessrespectively, V—potential value on the lower electrode, U is the applieddifference of potentials.

The boundary conditions required for solving the equation are listedbelow:

equality of the potentials on the dielectric plate surfaceV ₁(r,l)=V ₂(r,l),b<r<a V ₁(r,l)=V ₂(r,l)r<a  (2)V ₁(r,o)=V ₂(r,o)r<a  (3)

equality of the normal components of the inductions on the dielectricplate surface $\begin{matrix}\begin{matrix}{{ɛ\frac{\partial V_{1}}{\partial z}} = \frac{\partial V_{2}}{\partial z}} & {\quad{{b < r < a},{z = l}}} \\{{ɛ\frac{\partial V_{1}}{\partial z}} = \frac{\partial V_{2}}{\partial z}} & {\quad{{o < z < l},{r = a}}}\end{matrix} & (4)\end{matrix}$

conditions of potentials constancy and for the applied difference ofpotentialsV ₁(r,o)=V ₂(r,o)=V ₀ ,r<aV ₁(r,l)=V ₂(r,l)=V ₀ +U,r<b  (5)

As it is impossible to obtain analytical expressions the task is solvednumerically. The calculations are performed for the dimensions of theupper electrode b=2 cm, while the dielectric layer thickness is 3 mm.

The calculations were performed for the values of dielectric constant ε5and 20, and in this case there was not found any substantial dependenceof the field on that value. All the data is given rated to the value ofthe applied difference of potentials U. This allows obtaining the truefield strength value (its radial component) as a function of the appliedvoltage.

If the article being tested has no faults in the weld seam area, theelectric field is equipotential in the active zone. Therefore thedistribution of the electric potential per special programs wascalculated for the active zone section and is shown in colors in FIG.2,3,4 where the lines are drawn with a vector of equal potentials. Lowerin the same figures the results are shown of the numerical calculationfor the tangential component of the field along the radial coordinatefor

r=b on the dielectric plate surface. The designations on the axes ofFIG. 2,3,4 are identical for a) and b) and are shown on b).

An estimate of the input of the macroscopic polarization P₀ of thedielectric plate into the radial component of the electric fieldstrength tangential component.

In this case the dependence of the value P₀ on the applied field issupposed to be known (of course, it does not depend on the value of thedielectric constant ε in the dielectric, and moreover, the input of theinducted polarization part in P₀ should be neglected).

The potential of the polarized dielectric is known to be determined bythe expression (taking into regard the cylindrical symmetry):$\begin{matrix}{{\varphi( {r,z} )} = {\int{\int{\int{\frac{PR}{R^{3}}\quad{\mathbb{d}V^{1}}}}}}} & (6)\end{matrix}$

where R=r−r¹, r and r¹—the points of monitoring and outflow. By writingthis integral in the cylindrical system of coordinates and integratingone time with respect to z′ we obtain logarithmically divergentintegrals: $\begin{matrix}\begin{matrix}{{\varphi( {r,z} )} = {P\lbrack {{\int_{0}^{2\pi}{\int_{0}^{a}\frac{r^{\prime}\quad{\mathbb{d}r^{\prime}}\quad{\mathbb{d}\vartheta^{\prime}}}{\sqrt{r^{2} + r^{\prime 2} - {2{rr}^{\prime}\cos\quad\vartheta^{\prime}} + l^{2}}}}} -} }} \\{ {\int_{0}^{2\pi}{\int_{0}^{a}\frac{r^{\prime}\quad{\mathbb{d}r^{\prime}}\quad{\mathbb{d}\vartheta^{\prime}}}{\sqrt{r^{2} + r^{\prime 2} - {2{rr}^{\prime}\cos\quad\vartheta^{\prime}}}}}} \rbrack,}\end{matrix} & (7)\end{matrix}$

where 1—is the dielectric plate thickness. Using rather robustmathematical regularization methods that contain quite numeroussimplifications we shall obtain an expression for the radial componentof the field rated to the initial polarization value P₀ $\begin{matrix}{{E_{r}( {r,{z = l}} )} = {\delta\quad\frac{r}{r^{2} - a^{2} + {\beta(z)}}}} & (8)\end{matrix}$

where δ is a constant of the order of unity, and value β(z)<<1 As it isseen, when r approaches r to a the field value drastically increases. Itshould be noted that the set task was solved in the simplest version inorder to show the principal growth of E with the distance increasebetween the point of value E evaluation and the source where, as it isshown, it equals 0. As it is seen from (8), when r approaches a, thefield value increases much, while on the axis of the polarized plate itequals 0. Such behavior of the field is in full conformity with the tasksymmetry and proves the correctness of the obtained functionaldependence.

The physical mechanism of forming the tangentially directed discharge inthe tightness failure area is based on the article and polarized plateemission analysis of such charged particles as electrons and ions thatinduce the discharge microchannels. The type of the microchannels isdetermined by the operation of the electrons output, article geometryand the surfaces ratio of the polarized plate, positive electrode andthe article being tested. The latter ratio is chosen experimentally foreach article type, on the basis of the 7:1 criterion.

FIG. 6 shows artificial defects 1 and actual faulty fusion defects 2 butwith an opening below 100 μm. These defects were not rated according totheir size, but when their conventional diameter is below 100 μm it ischaracterized by the absence of a discharge that causes an exposure ofthe X-ray film. In the photo picture light traces of air are seenproving that it leaked through these faulty fusion areas into thepreliminary evacuated welded volume.

Due to the proposed invention the tightness testing method that is beingapplied permits to reduce the testing labor content, to expand the rangeof its usage for the articles for which any introduction of foreignmatter into the working medium is inadmissible (for instance, gaspurging), and to provide a new useful quality—the possibility of a gasdischarge visualization of tangentially oriented faults with littleopening in thin layer weld seams.

The method of nondestructive non-contact tightness testing based on gasdischarge visualization is practiced in the following manner:

Mode One:

the surface area of a flat controlled power source is calculated and aflat metallic positive electrode is selected taking into regard thattheir surface areas to have the ratio 7:1.

Mode Two:

a dielectric electrode is selected proceeding from the ratio of itssurface area and the metallic electrode surface area to be 1.2; 1, andit is placed on the metallic electrode symmetrically to the center ofthe positive electrode so that the positive electrode is to becompletely covered.

Mode Three:

the electrode is subjected to a series of high voltage pulses thatpolarize the dielectric plate.

Mode Four:

the polarized plate is covered with an X-ray film placed in a blackpaper pack with the emulsion layer facing the surface of the articlebeing tested.

Mode Five:

on the X-ray film in its center the article to be tested is placed, itsmetallized surface is earthed, and a series of high voltage pulses isapplied to its positive electrode.

Mode Six:

subsequently the film is taken out of the pack and subjected toconventional photoprocessing.

In accordance with the above listed operations and modes the method wastested on samples of the articles containing inside a multiplayerworking medium in the shape of rectangular briquettes welded into ametallized polyethylene film. The width of the weld seam was 5 mm, thefilm thickness was 0.2 mm. Faulty tightness was imitated by welding inthe weld seam of 100-300 mm diameter steel wire pieces which were thentaken out of the weld seam. A practical study permitted to optimize theprocess parameters of the preliminary polarization of a dielectricplate, and it was found that the most optimal is polarization of thedielectric plate by not less than three pulses of 100-120 μsec durationmodulated by a 200-300 kHz frequency.

The presence of defects on the photographic picture (FIG. 6) wasdetermined by an expert appraisal performed by certified flaw detectionoperators. The diameter of the minimum detectable faulty fusion exposedby avalanche discharges was 100 μm.

Thus, the proposed method permits to detect ≧100 μm faulty fusiondiscontinuities in thin layer weld seams, thus providing tightnesstesting of articles with a working medium welded into a metallizedpolyethylene film in the process of manufacturing, during finalizing oftechnological processes, storage and operation.

The basic modes and operations of the proposed method will permit todetermine the tightness of self-contained chemical power sources andother devices of similar design concepts thus allowing to obtain a newuseful effect.

REFERENCES

1. Korotkov K. G. Studies of physical processes during gas dischargevisualization of various objects./Abstr. Diss Cand. Sc.(Physics).-Leningrad.:1982/. (In Russian).

2. Ziunov V. G., Korkin Yu. V. Analytical review of literature on gasdischarge visualization (Kirlian method), NPAN USSR, dep. VINITI, 1985,No. 4117-85. (In Russian).

3. Bankovsky N. G., Korotkov K. G., Petrov N. N. Physical processes offorming images during nazodischarge visualization.//Radiotekhnika Ielectronika.V.31, issue 4, 1986. (In Russian).

4. Method of tightness testing of articles.-N. G. Bankovsky and K. G.Korotkov.-Inventor's Certificate USSR No. 1290120, 1987, G 01 M 3/40,15.02.1987.-Bull. No. 6. (In Russian).

5. Method of nondestructive testing.-S. F. Romaniy.-Inventor'sCertificate USSR No. 781687, G 01 N 27/24, 23.11.1980.-Bull. No. 43. (InRussian).

6. Method of nondestructive testing.-S. F. Romaniy, Z. D.Cherny.-Inventor's Certificate USSR No. 949484. G 01 N 27/82,07.08.1982, Bull. N. 29. (In Russian).

7. Method of high frequency recording of blow-holes in a solidmaterial.-S. F. Romaniy, V. S. Karpenko, V. V. Burminov.-Inventor'sCertificate USSR No. 1091106, G 03 B 41/00.-1984, Bull. No. 17. (InRussian).

8. Romaniy S. F., Cherny Z. D. Nondestructive testing of materials usingKirlian's method. Dnepropetrovsk, DGU Publishers, 1991, 144. (InRussian).

1. A method of nondestructive noncontact tightness testing based on gasdischarge visualization comprised of subjecting the article being testedto a high voltage pulsed electric field formed between the positiveelectrode and the metallic layer of a multi-layer article casing,recording on a photo carrier of an image of the gas discharge processthat is taking place, wherein the article is placed on a dielectricplate arranged on a positive electrode the surface area of which isselected in the ratio to the article surface area not less than 7:1, andthe article is additionally subjected to an electric field of apreliminary polarized dielectric plate arranged between the positiveelectrode and the article.
 2. A method of nondestructive noncontacttightness testing according to p. 1, wherein the ratio between thesurface area of the dielectric plate and the surface area of thepositive electrode is selected within 1.2:1, and the plate is arrangedsymmetrically relative to the positive electrode center.
 3. A method ofnondestructive noncontact tightness testing according to p. 1, whereinthe dielectric plate is preliminary polarized by not less than threehigh voltage pulses.
 4. A method of nondestructive noncontact tightnesstesting according to p. 1, wherein polarization of the dielectric plateis effected by 100-120 μsec duration pulses modulated by 200-300 kHzfrequency.